1. Field of the Invention
This invention relates generally to lithography, and more specifically to the protection and transport of lithographic reticles.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A semiconductor wafer, for example, can be used as a substrate to fabricate an integrated circuit.
During lithography, a reticle is used to transfer a desired pattern onto a substrate. The reticle is formed of a material transparent to the lithographic wavelength being used. For example, in the case of visible light, the reticle would be formed of glass. The reticle has an image printed on it. The size of the reticle is chosen for the specific system in which it is used. During lithography, a wafer, which is supported by a wafer stage, is exposed to an image projected onto the surface of the wafer corresponding to the image printed on the reticle.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. After exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer. As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer.
In addition to the transmissive reticles just described, reflective reticles are also used in the art. For example, reflective reticles are used for short wavelength light that would otherwise be absorbed by a transmissive glass reticle.
In an effort to keep contamination of the reticle surface to a minimum, lithography processing is performed in a xe2x80x9cclean room.xe2x80x9d A clean room is an enclosure having a specified controlled particle concentration. In order to maintain the specified controlled particle concentration, gaseous materials are provided to and removed from the enclosure. A considerable amount of expense is associated with maintaining a clean room. This expense is related, in part, to the size of the clean room and the equipment needed to maintain it. For example, as reticles are transported from one stage in a lithographic process to another, they are susceptible to contamination due to particles found within the processing area. To minimize the potential for contamination, the entire room in which the reticle is transported is usually maintained in a clean state. Thus, there is an incentive to reduce the environment that must be maintained in the clean state. A further incentive for reducing the size of the clean room is safety. In some cases, clean rooms are oxygen deficient and therefore unfit. for human occupancy. If the clean room can be isolated to a smaller environment, then the surrounding area can be maintained for safe use and occupancy by humans.
U.S. Pat. No. 6,239,863 issued to Catey et al., May 29, 2001, and commonly assigned to Silicon Valley Group, Inc. (incorporated by reference herein in its entirety) discloses a removable cover for protecting a reticle used in a lithography system. The removable cover includes a frame and a membrane supported by the frame. The removable cover can further include at least one reticle fastener that applies force to the reticle, thereby preventing movement of the removable cover relative to the reticle when the removable cover is in place. The use of the reticle fastener presents an opportunity for contamination from the contact between the reticle and the reticle fastener.
Thus, what is needed is a way of further reducing the potential for reticle contamination during transport. while also reducing the need to maintain cleanliness in a much larger volume within an enclosure or in a large environment.
The present invention provides a reticle protection and transport system and method for a lithography tool. The system includes an indexer that stores a plurality of reticles and a removable reticle cassette. The removable reticle cassette comprises an inner chamber and an outer chamber. The system further includes an end effector coupled to a robotic arm. The end effector engages one of the plurality of reticles to enable the reticle to be positioned within the removable reticle cassette and thereafter transported. To further protect the reticle within the removable reticle cassette, the system further includes a seal coupled to the end effector and the robotic arm.
To transport the reticle, the reticle is first loaded onto the end effector. Next, the end effector is used to create an arrangement wherein the reticle is loaded into the removable reticle cassette. Importantly, the reticle and removable reticle cassette do not come into contact with each other. The arrangement is then sealed and transported from the indexer to the mount for performing lithographic exposure. Once lithographic exposure is completed, the arrangement is returned to the indexer, where the reticle is withdrawn from the removable reticle cassette and stored.
These and other advantages and features will become readily apparent in view of the following detailed description of the invention.